That is the correct answer and it is good to see someone not posting answers willy-nilly without knowing exactly what is happening. As 1080i is interlaced, fast moving scenes can result in a stepping effect with every other horizontal line being out of sync.

As both 720p and 1080i are requirements of the HD Ready specification, all HD Ready certified TVs support both formats including those with lower resolution screens. At close range, a 1080 resolution picture certainly is more noticeable but at longer viewing distances it is much more difficult to see the difference and in fact, there is a threshold by which that the difference is not noticeable to the human eye with regards to clarity, although the stepping effect can still be seen at further distances.